Normally, light waves vibrate in a large number of planes about the axis of a light beam. If the waves vibrate in one plane only, the light is said to be plane polarized. Several useful optical ends and effects can be accomplished by plane polarized light. For example, in the manufacture of electrooptical devices, such as liquid crystal display screens, crossed polarizers can be used in conjunction with an addressable liquid crystal interlayer to provide the basis for image formation. In the field of photography, polarizing filters have been used to reduce glare and the brightness of specular reflection. Polarizing filters have also been used for the reduction of glare on CRT display monitor screens.
While several materials possess to a degree inherent polarizing properties, synthetic polarizing materials based on thin polymeric films are desirable for their comparative ease of manufacture and handling, their ability to be tailored for particular uses, and the comparative ease with which they may be incorporated into desired end products.
The production of linear light polarizing films has been well described in the art. Linear light polarizing films, in general, owe their properties of selectively passing radiation vibrating along a given electromagnetic radiation vector (and absorbing electromagnetic radiation vibrating along a second given electromagnetic radiation vector) to the anisotropic character of the transmitting film medium.
Dichroic polarizers are linear polarizers of an absorptive variety that owe their light-polarizing capabilities to the vectorial anisotropy of their absorption of incident light waves. The term "dichroism" is used herein as meaning the property of differential absorption of the components of an incident beam of light, depending upon the vibration directions of said component. Thus, light entering a dichroic film encounters two different absorption coefficients--one low and one high. The emerging light vibrates predominantly in the direction of low absorption.
Among the several varieties of synthetic linear light polarizers are the so-called "K Sheet"-type polarizers, which are characterized by their comparatively good environmental resistance to heat and humidity. As described in U.S. Pat. Nos. 2,173,304 and 2,306,108, both issued to E. H. Land and H. G. Rogers, "K Sheet"-type polarizers are usually prepared by at least partially dehydrating a hydroxylated polymer, such as polyvinylalcohol. In particular, before or after a molecular orientation step (i.e., stretching), a sheet of polyvinylalcohol is dehydrated by exposure in an elevated temperature environment to fumes of a very strong acid catalyst; water molecules are liberated, thereby forming a polymer with conjugated vinylene blocks. In the resultant sheet, incident light is linearly polarized by transit through the oriented polyvinylene molecules.
While useful "K Sheet"-type polarizers can be made by conventional fuming processes, in certain processing environment, and in the absence of suitable controls, said processes can result in polarizers having present a degree of "streaking" and "mottling" unsuitable or undesirable for certain applications, particularly those requiring a high level of optical precision. One condition believed to contribute to these artifacts are environmentally-influenced fluctuations in acid fuming that result in a less than uniform catalytic dehydration. For example, even in a hermetically-sealed enclosure, the elevated temperatures of a fuming oven can produce thermal currents that disrupt or otherwise interfere with uniform fuming. In light of such potential variability, need is present for a method for the manufacture of a polyvinylene-based polarizer that reduces reliance--or more preferably does not involve--the step of acid fuming.